Hepatitis C virus (HCV) can cause acute and chronic hepatitis and is also a major cause of hepatocellular carcinoma. This virus has a positive-stranded RNA genome which encodes a polyprotein of approximately 3,010 amino acids. The core protein is located at the amino terminus of the polyprotein sequence. The HCV core protein sequence can produce two major products with length of 191 a.a. (P21) and 173 a.a. (P19), respectively, and a minor product with a length of approximately 151 a.a. These core proteins could bind to the 5' untranslated region (UTR) of the HCV genome. This binding may be important for regulating translation, replication and/or packaging of the HCV genomic RNA. The first three specific aims of this proposal is to continue our previous studies to examine the mechanism that regulates the expression of the core proteins, the role of individual core proteins in the life cycle of HCV, and how and why the core protein binds to the 5'UTR of the HCV genome. We have recently identified a deletion mutant of the HCV genome. This finding raises the possibility that it may be feasible to perform deletion mapping experiments to characterize the cis-acting elements required for the replication of the HCV genome. The fourth specific aim of this proposal is to investigate this possibility. The fifth specific aim is to investigate the possible functions of cellular protein factors which could bind to the 5'UTR of the HCV genome. The goal of this aim is to investigate the biological meanings of this finding. Thus, there are a total of five specific aims in this application. These specific aims are designed to continue our present research to study the replication cycle of HCV. We believe the knowledge gained from our studies will eventually improve the diagnosis and treatments for the diseases caused by HCV.